Video details
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Key takeaways:
- Rolling shutter is impressive (sub 10ms) in all video modes
- Readout rate is a fraction too slow to allow 120, hence the need for a slight crop
- Sensor has a dual conversion gain design, switching over 4.66 EV above base ISO
Rolling shutter
The a7S III takes a different approach to the one used in more stills-focused generalist cameras: rather than taking advantage of a high pixel count sensor to give more detailed footage, the a7S III uses just a handful more than the bare minimum required to shoot 4K at all. The great advantage of this approach is that it has less data to read out. Which in turn means less risk of rolling shutter.
We measured the camera in a variety of modes, and the decision appears to pay off.
| Video mode | Rolling shutter rate |
|---|---|
| 4K 60p / 30p / 24p | 8.7ms |
| 4K 120p | 7.7ms |
| 4K Steady Shot Active (60p / 30p / 24P) |
7.8ms |
It's worth noting that the rolling shutter rate for the full sensor width modes are a fraction short of the 8.3ms (1/120 sec) needed to shoot at 120 frames per second. This explains why a crop is needed to deliver 4K/120p but also why that crop doesn't need to be too big.
Sub 10ms rolling shutter rates are generally considered excellent and will only begin to show visibly skewed verticals with very fast moving subjects.
We haven't yet tested rolling shutter for the camera's 16-bit Raw output mode. Sensors are able to read-out faster if they are sampling the data at lower bit-depths, so there's the possibility that 16-bit mode will show more rolling shutter if the internal video modes are captured in 14-bit precision. We hope to check this week.
Dual Gain
As is common with modern large sensors cameras, the a7S III has a dual gain sensor. Sony doesn't draw attention to this, either in its marketing or in the camera's interface but most of its large sensors dating back to the original a7S have had a dual gain design.
As usual this means switching from the default, high dynamic range readout circuit in the pixel to a lower read noise (and lower DR) readout method for higher ISOs.
Because the ISO rating of each camera mode depends on the tone/gamma curve, the ISO value at which the switch is made depends on which Picture Profile you use. Our tests suggest the camera moves to its higher gain step 4.33EV above the base setting (the same amount above base the a7S and a7S II).
| Base ISO | 2nd gain step | |
|---|---|---|
| Standard color (Picture Profile Off) |
ISO 80 | ISO 1600 |
| S-Log (2 or 3) | ISO 640 | ISO 12,800 |
As with previous Sony models, the camera doesn't in any way acknowledge the switchover, but if you're working near the ISO where the camera changes modes, you might be better-off moving up to the higher gain, lower read-noise state.
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